This invention relates to a display device having a first electrode formed on a substrate and a second electrode provided facing this.
Light, thin, so-called flat panel displays are the subject of attention as image display devices (displays) to replace bulky, heavy, cathode ray tubes.
Liquid crystal displays (LCDs) are popular as flat panel displays, and electrochromic displays (ECDs) are another example of similar non-luminescent displays, while as examples of the luminescent displays which have recently become a focus of attention there are plasma display panels (PDPs) and electroluminescent displays (ELDs). Amongst the electroluminescent displays, high brightness may be obtained with, in particular, organic electro-luminescent displays and there is considerable research and development in this area due the fact that a full colour display is possible.
These flat panel displays are operated by applying a voltage, or by passing current, between facing first and second electrodes. In such circumstances, since electric charge concentration readily occurs at the edge regions of electrodes with a small radius of curvature, undesirable phenomena such as dielectric breakdown and leakage currents tend to occur at the edge regions.
In order to suppress these phenomena, covering the edge regions of the first electrodes with an insulating layer is known. In this way, it becomes possible to mitigate electric field concentration at the electrode edge regions. Furthermore, in SP-6222315, there is disclosed a technique for further resolving the aforesaid problem by making the thickness of the insulating layer at the boundary portion where the first electrode is exposed by the insulating layer gradually increase with distance from the boundary, or in other words by giving the cross-section a tapering shape, so that there is smooth build-up of the organic thin film layer and second electrode produced by film formation after the forming of the insulating layer.
Generally speaking, polyimides are used as the insulating layer, and non-photosensitive, negative photosensitive and positive photosensitive types are known.
In the case of a non-photosensitive polyimide, in the patterning of the insulating film numerous photolitho processes are required, namely application of the polyimide precursor onto the substrate, prebaking of the polyimide precursor (also referred to as drying or semi-curing), application of a photoresist onto the polyimide precursor, baking of the photoresist (also known as drying or prebaking), exposure of the photoresist, development of the photoresist, etching of the polyimide precursor, elimination of the photoresist, and curing of the polyimide precursor (also referred to as post baking). Consequently, there is the problem that the process is complex and the yield poor.
Furthermore, in order to give the insulating film cross-section a tapering form, it is necessary to optimise various parameters such as the photoresist development conditions and the polyimide precursor etching conditions, and there is the problem that the setting of such conditions is complex.
If a photosensitive type polyimide such as a negative or positive type photosensitive polyimide is used, then patterning of the insulating layer is possible without using a photoresist, and therefore it is possible to overcome the problems of process complexity and poorness of yield. However, with a negative type photosensitive polyimide, instead of a direct tapering shape there tends to be formed an undercut shape or rectangular shape, so no effect is obtained in mitigating electric field concentration at the edge regions. Furthermore, with regard to positive type photosensitive polyimides, while JP-A-8-171989 discloses a technique for introducing an o-nitrobenzyl ester group at the polyamic acid carboxyl group, this positive type photosensitive polyimide has the problem of poor pattern processability, and it is not possible to carry out fine patterning as in the present invention.
The present invention has the objective of carrying out the patterning of an insulating layer comprising polyimide by a simple process. Furthermore, it has the objective of readily obtaining the tapered shape which is the desired cross-section of the insulating layer.
The present invention relates to a display device which is a display device which includes a first electrode formed on a substrate, an insulating layer formed on the first electrode in such a way that the first electrode is partially exposed, and a second electrode provided facing the first electrode, and the insulating layer is a positive-type photosensitive polyimide in which polymer having, as its chief component, structural units represented by the following general formula (1), and a photoacid generator, are the indispensable components. 
(R1 and R2 represent divalent to octavalent organic groups having at least two carbon atoms, and R3 and R4 represent hydrogen, an alkali metal ion, an ammonium ion or an organic group with from 1 to 20 carbons. R3 and R4 may be the same or different. m is an integer in the range 3 to 100,000, and n and o are integers in the range 0 to 2. p and q are integers in the range 0 to 4, and n+q greater than 0.)